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September 18, 2019 – Showers and a chance of thunderstorms are expected across the region today as another weather system moves through. Showers will linger across the mountains on Thursday, then dry and milder weather is expected heading into the weekend.

Discussion

Satellite imagery shows front moving slowly inland (southeastward) from NW California. Radar indicates some light showers across the northern Sacramento Valley and mountains to the west and north of Redding. Skies remain mostly clear to the south across the southern two thirds of the forecast area. Current temperatures vary from the 30s in the colder cloud-free mountain valleys ahead of the front, to the upper 60s across milder portions of the Central Valley.

Little change to forecast. Front spreads showers south to about the Sacramento area this morning before slowing across the northern San Joaquin Valley this afternoon, with scattered showers and thunderstorms expected later this morning into this evening (mainly Sacramento area northward) with the upper trough and approaching vort. QPF expected to vary widely given the convective nature of the system with amounts varying from a few hundredths of an inch in the valley to locally a half inch or more over the northern Sierra.

The convective potential is still on track with model forecast soundings indicating sufficient CAPE in the afternoon for potential for thunder mainly N of the SAC area, per the SPC outlook. HREF indicates a high probability of >40 dBZ echoes over the W slope Sierra in the afternoon which could correlate to heavy showers lowering snow levels to pass levels once again. Motorists should be alert for sudden bursts of snow over the high trans-Sierra passes in the afternoon.

Model agreement in timing and position of a vort max moving through the upper trof in the afternoon and the NAM REFL prog suggests ingredients coming together for possible rotating storms in the “Chico (area) convergence zone.” Similar to Mon, the low level shear is weak (light winds), but the upper level vort max, if forecast with any reasonable accuracy, would augment the mid level cyclonic flow possibly yielding a lower end mesocyclone.

Unsettled weather should exit most of the region on Thursday with skies clearing out from west to east. The exception would be the Sierra where cyclonic flow is forecast to linger for most of the day causing some wrap around moisture and showers. With the mean trough centered over Nevada, meridional height gradients set up supporting a period of dry, northerly flow over the Valley on Thursday and continuing into Friday. Wind fields do not appear particularly strong although gusts could 15-20 mph or so on Thursday afternoon and evening. A continued moderation in the temperatures will accompany this shift in the winds with highs in the mid 80s in the Vly on Friday. This is still 1 to 9 degrees below average except in the Carquinez Strait where readings will be slightly above normal.

Extended Discussion (Sunday through Wednesday)

Progressive Pacific flow will usher in another upper trough which is set to arrive on the West Coast by Sunday morning. The upstream height falls ultimately shift the transient ridge into the Rockies by the afternoon. Some increase in cloud cover is likely over sections of interior northern California as high-level moisture streams off the Pacific. However, Valley highs should have no issue reaching the upper 80s which is close to climatology for mid-September. A few scattered showers will be possible over the mountains of northern Shasta County extending into western Plumas County as well the northern Sierra crest.

The biggest question mark ahead is where this trough situates itself as it will dictate whether the pattern will be unsettled or possibly dry and breezy. Deterministic models continue to waver with the 06Z GFS and 00Z ECMWF taking an upper low along the California/Nevada border into the Desert Southwest. Ensemble spaghetti plots show plenty of diversity in its solutions to support tracks west and east of this scenario. Will continue to maintain a period of northerly flow early next week which ensures Valley highs stay in the upper 80s to low 90s. The parent upper low potentially stalls in the southwestern U.S. for part of next week which would dictate the prevailing flow aloft. While not in the forecast, the 00Z ECMWF shows downsloping northeasterly flow which suggests highs in the upper 90s early next week. So there is some room for temperatures to rise higher than currently in the forecast.